Vertex Paint - creating gradient from selected vertices?
To make the gradient as the result of the vertex paint assign a texture as a pattern to the brush or use weight paint and convert it into the vertex painted colors.

Painting vertex colors with brush pattern


In Edit mode select faces which you'd like to be affected by Vertex paint. Go to Vertex Paint mode and enable the Face Selection Masking option to make the selected faces be the only ones affected.
In the brush settings, Texture rollout assign texture with desired effect. If the resulted vertex colors should be gradient-like, procedural texture of the Blend type will be good enough.
Choose Stencil option to paint with texture in the same way as in Texture Painting. Move it in the 3D Viewport with RMB to set up painting as desired. 




Converting Weight Paint into desired vertex colors

There's an addon for converting weight paint into vertex colors and vice versa.


After activating it, create a weight paint to be converted. Use Weight Gradient, Quantize tools etc. from the Toolshelf to create the final result. 
In the Vertex Colors mode, 3d Viewport header find and run Paint > WeightToVertexColor.